Weathering is the business of making a model look used, and it is where scale modelling and miniature painting share the most technique. Weathering paint covers the effect paints that produce rust, grime and chipping; weathering powders are dry pigments dusted on and fixed in place. Both are easy to apply and easy to overdo, and the difference is almost entirely about restraint and placement.
Placement is the whole technique
Wear happens where things touch, and dirt collects where water runs and settles. That means chipping belongs on edges, hatches, footplates and anywhere a hand or a boot goes, and dust belongs low down, in recesses and behind anything that throws it up. A model weathered uniformly across every surface reads as dirty rather than used, because nothing in the real world wears evenly. Deciding where before deciding how much is what separates convincing weathering from a brown model.
Powders, and fixing them in place
Weathering powders are finely ground pigment with no binder, which is what lets them be dusted, blended and wiped back until the effect is right. That freedom ends when they are fixed: a fixative or a varnish locks them down, and it usually darkens them at the same time, so the intensity you want is the intensity after fixing rather than before. Powders applied over a matt surface grip far better than over gloss, and a model varnished matt before weathering will hold them where a glossy one will not.
Order of operations
The reliable sequence is paint, gloss varnish, washes and panel lines, decals if any, matt varnish, then weathering paint and powders, then a light fixing coat. Gloss under washes lets them flow rather than stain; matt over the top gives powders something to grip. Working out of order is the usual reason weathering looks wrong for no obvious reason, and it is much harder to correct than to plan, because most weathering steps are difficult to remove once the surface underneath has been sealed.
Questions people ask about weathering paint
What is the difference between weathering paint and weathering powders?
Weathering paints are liquid effect paints for rust, grime and chipping. Powders are dry pigment dusted on, blended, and then fixed with a varnish or fixative.
Where should I put weathering on a model?
Where wear and dirt actually happen: chipping on edges and contact points, dust low down and in recesses. Uniform weathering reads as dirty rather than used.
Do weathering powders need sealing?
Yes, or they rub off. Fixing usually darkens them, so judge the intensity you want as the intensity after the fixing coat, not before.